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"Osu" as a greeting (contracted from "Ohayossu", contracted from "Ohayou gozaimasu"), is written in hiragana おっす, and as Yawata-san suggested, can vary by region. It's like "Hiya" or "Howdy".

"Osu" as a response in the dojo is written with ateji kanji (kanji with irregular readings) 押忍. It's essentially a different word from the other (although they probably came from the origin) in use and orthography. They look the same in romanization, but they are two different words.

Josh Reyer

The lyf so short, the crafte so longe to lerne,
Th'assay so harde, so sharpe the conquerynge...
- Chaucer

I found an article on the web in Japanese some time ago that mentioned that "osu" originated in the Japanese Navy, among the new recruits. It was used, as others mentioned before, as a shortened "ohayo gozaimasu." It morphed into a general reply to orders, as well as a general greeting. It was not considered polite at that time, but it has become a general, multi-purpose greeting or reply. It has sort of a "tough-guy" ring to it in general use in Japan, as someone mentioned earlier, often used in sports clubs etc.

I believe that the use of "osu" depends on the feeling of the dojo, and the dojo-cho.

I remember way back, Inagaki Sensei from the Iwama Dojo often brought his students from a university aikido club to train at the Iwama Dojo. They all used "osu" as a general reply.

Saito Sensei would tell students that "osu" wasn't used in the dojo.

I remember him saying this to one group of students of Inagaki Sensei's, who proceeded to reply with a whole hearted, and unified "OSU!"

When I had the distinct privledge to spend 10 days on a Japanses Destroyer many moons ago. Osu was used as a greeting in a very informal way. Among friends. I asked the meaning and the answer was basically lazy slang for ohayo gozaimasu.

I can only speak from my 40 years experience in Budo. I started in the early 1970's with JKA style Shotokan Karate.. The expression OSS was, and still is, used to mean a number of things like, I understand, yes, hello, goodbye, Karateka use the term to end letters, emails, phone calls etc.

It is much like the motto of the Irish regiments that use Faugh a Ballagh (clear the way). current serving and veteran soldies will use that term to end letters, emails etc. Manny time using FaB instead of writing it iut. The Irish Guards use their motto Quis Saparabit (who will seperate us ?) in much the same way. Often just written as QS.
Most people know the Irish origin of the American military regiment using the motto Garryowen..
In my Aikido training oss was never used. It is not used in Kendo or in any koryu that I know of.
There is no harm in using the term as long as you use it in the spirit of the art.

It is much like the motto of the Irish regiments that use Faugh a Ballagh (clear the way). current serving and veteran soldies will use that term to end letters, emails etc. Manny time using FaB instead of writing it iut. The Irish Guards use their motto Quis Saparabit (who will seperate us ?) in much the same way. Often just written as QS.
Most people know the Irish origin of the American military regiment using the motto Garryowen..
In my Aikido training oss was never used. It is not used in Kendo or in any koryu that I know of.
There is no harm in using the term as long as you use it in the spirit of the art.

our motto is "Oh Shit!" (Crap!). this usually happened during our atemi phase of training, which is 100% of the time or approximately close to it.